Oxiranyl-acyl derivatives as additives for electrolytes in lithium ion batteries

ABSTRACT

The present invention relates to an electrolyte composition (A) containing (i) at least one aprotic organic solvent; (ii) at least one conducting salt; (iii) at least one compound of formula (I) and (iv) optionally at least one further additive.

The present invention relates to an electrolyte composition (A) containing

-   (i) at least one aprotic organic solvent; -   (ii) at least one conducting salt; -   (iii) at least one compound of formula (I)

-   -   wherein     -   X, R¹, R², R³, R⁴, and n are defined below, and

-   (iv) optionally at least one further additive.

The present invention further relates to the use of compounds of formula (I) as additives for electrolytes in electrochemical cells and to electrochemical cells comprising the above described electrolyte composition (A), at least one cathode (B) comprising at least one cathode active material, and at least one anode (C) comprising at least one anode active material.

Storing electrical energy is a subject of still growing interest. Efficient storage of electric energy would allow electric energy to be generated when it is advantageous and used when needed.

Accumulators, for example lead accumulators and nickel-cadmium accumulators, have been known for many decades. The known lead accumulators and nickel-cadmium accumulators have the disadvantages, however, of a comparatively low energy density and of a memory effect which reduces the rechargeability and hence the useful life of lead accumulators and nickel-cadmium accumulators.

Lithium ion accumulators, frequently also referred to as lithium ion batteries, are used as an alternative. They provide higher energy densities than accumulators based on lead or comparatively noble heavy metals.

Since many lithium ion batteries utilize metallic lithium or lithium in oxidation state 0, or produce it as an intermediate, they are water sensitive. Moreover, the conductive salts used, for example LiFP₆, are water sensitive during long-term operation. Water is therefore not a usable solvent for the lithium salts used in lithium ion batteries. Instead, organic carbonates, ethers, esters and ionic liquids are used as sufficiently polar solvents. Most state of the art lithium ion batteries in general comprise not a single solvent but a solvent mixture of different organic aprotic solvents.

During charge and discharge of lithium ion batteries various reactions take place at different cell potentials. It is known that during the first charging process of a lithium ion battery usually a film is formed on the anode. This film is often called solid electrolyte interface (SEI). The SEI is permeable for lithium ions and protects the electrolyte from direct contact with the anode and vice versa. It is formed by reductive decomposition of components of the electrolyte composition like solvents, e.g. carbonates, esters, and ethers, and conductive salts on the surface of the anode, especially if the anode active material is a carbonaceous material like graphite. A certain amount of the lithium present in the cell is irreversibly consumed for the formation of the SEI and cannot be replaced. One possibility to reduce the amount of irreversibly consumed lithium is the addition of suitable chemical compounds which are easily decomposed on the anode by reduction and thereby forming a film on the surface of the anode. One especially well suited compound is vinylene carbonate, see for instance EP 0 683 587 B1 and U.S. Pat. No. 6,413,678 B1. Vinylene carbonate forms a stable SEI on a graphite anode in lithium ion batteries.

Other film forming additives are known, inter alia oxiranyl derivatives as described in US 2009/0035656 A1 disclosing glycidyl ether compounds as film forming additives for electrolytes of lithium batteries.

Nevertheless there is still the need for enhancing the lifetime of secondary batteries and a demand for electrolyte additives leading to a prolonged life time and cycle stability of secondary electrochemical cells.

It was an object of the present invention to provide an electrolyte composition leading to an improved lifetime of electrochemical cells, in particular lithium ion batteries. A further object of the present invention was to provide electrochemical cells, in particular lithium ion batteries of high energy density and/or higher operating voltage having good performance characteristics and long lifetime.

This object is achieved by an electrolyte composition (A) containing

-   (i) at least one aprotic organic solvent; -   (ii) at least one conducting salt; -   (iii) at least one compound of formula (I)

wherein

-   -   X is either carbon, S(O) or P(OR⁸);     -   n can be 0 or 1;     -   R¹ is selected from C₁-C₆ alkyl, C₃-C₆ (hetero)cycloalkyl, C₂-C₆         alkenyl, C₃-C₆ (hetero)cycloalkenyl, C₂-C₆ alkynyl, C₅-C₇         (hetero)aryl, and C₆-C₁₃ aralkyl, wherein alkyl,         (hetero)cycloalkyl, aralkyl, alkenyl, (hetero)cycloalkenyl,         alkynyl, and (hetero)aryl may be substituted by one or more         substituents selected from F, CN, C₁-C₆ alkyl, C₃-C₆         (hetero)cycloalkyl, C₂-C₆ alkenyl, C₅-C₇ (hetero)aryl, OR⁵,         C(O)R⁵, C(O)OR⁵, OC(O)R⁵, OC(O)OR⁵, OC(O)C(O)OR⁵, OS(O)₂R⁵, and         S(O)₂OR⁵;     -   R², R³ and R⁴ are independently from each other selected from H,         C₁-C₆ alkyl, C₃-C₆ (hetero)cycloalkyl, C₂-C₆ alkenyl, C₃-C₆         (hetero)cycloalkenyl, C₂-C₆ alkynyl, C₅-C₇ (hetero)aryl, and         C₆-C₁₃ aralkyl, C(O)R⁶, C(O)OR⁶, and S(O)₂OR⁶, wherein alkyl,         (hetero)cycloalkyl, aralkyl, alkenyl, (hetero)cycloalkenyl,         alkynyl, and (hetero)aryl may be substituted by one or more         substituents selected from F, CN, C₁-C₆ alkyl, C₃-C₆         (hetero)cycloalkyl, C₂-C₆ alkenyl, C₅-C₇ (hetero)aryl, OR⁷,         C(O)R⁷, C(O)OR⁷, OC(O)R⁷, OC(O)OR⁷, OC(O)C(O)OR⁷, OS(O)₂R⁷, and         S(O)₂OR⁷; or R² and R³ are combined with each other and form         together with the C—C single bond of the oxirane cycle a 5 to 7         membered hydrocarbon cycle which is substituted by at least one         group selected from (O) and oxiranyl, and/or two adjacent         C-atoms of the hydrocarbon carbon cycle form together with an         O-atom an additional oxirane cycle, and R⁴ is defined as above;     -   R⁵, R⁶, R⁷ and R⁸ are independently from each other selected         from H, C₁-C₆ alkyl, C₃-C₆ (hetero)cycloalkyl, C₂-C₆ alkenyl,         C₃-C₆ (hetero)cycloalkenyl, C₂-C₆ alkynyl, C₅-C₇ (hetero)aryl,         C₆-C₁₃ aralkyl, wherein alkyl, (hetero)cycloalkyl, aralkyl,         alkenyl, (hetero)cycloalkenyl, alkynyl, and (hetero)aryl may be         substituted by one or more substituents selected from F, CN, and         oxiranyl; and

-   (iv) optionally at least one further additive.

The problem is further solved by the use of at least one compound of formula (I) as additive for electrolytes in electrochemical cells; and by the electrochemical cell comprising the electrolyte composition (A) as described above, at least one cathode (B) comprising at least one cathode active material, and at least one anode (C) comprising at least one anode active material.

The compounds of general formula (I) show high reduction potentials indicating their suitability as additives forming a film on the anode of secondary electrochemical cells. Electrolytes for lithium ion secondary batteries comprising at least one aprotic organic solvent or a mixture thereof, at least one conducting salt leads and at least one compound of general formula (I) show good values of the internal resistance and very small decrease of capacity retention during cycling.

The inventive electrolyte composition (A) is preferably liquid at working conditions; more preferred it is liquid at 1 bar and 25° C., even more preferred the electrolyte composition is liquid at 1 bar and −15° C., in particular the electrolyte composition is liquid at 1 bar and −30° C., even more preferred the electrolyte composition is liquid at 1 bar and −50° C.

The electrolyte composition (A) contains at least one aprotic organic solvent (i), preferably at least two aprotic organic solvents (i). According to one embodiment the electrolyte composition (A) may contain up to ten aprotic organic solvents (i).

The at least one aprotic organic solvent (i) is preferably selected from

-   (a) cyclic and acyclic organic carbonates, which may be partly     halogenated, -   (b) di-C₁-C₁₀-alkylethers, which may be partly halogenated, -   (c) di-C₁-C₄-alkyl-C₂-C₆-alkylene ethers and polyethers, which may     be partly halogenated, -   (d) cyclic ethers, which may be partly halogenated, -   (e) cyclic and acyclic acetals and ketals, which may be partly     halogenated, -   (f) ortho esters, which may be partly halogenated, -   (g) cyclic and acyclic esters of carboxylic acids, which may be     partly halogenated, -   (h) cyclic and acyclic sulfones, which may be partly halogenated, -   (i) cyclic and acyclic nitriles and dinitriles, which may be partly     halogenated, and -   (j) ionic liquids, which may be partly halogenated.

More preferred, the at least one aprotic organic solvent (i) is selected from cyclic and acyclic organic carbonates (a), di-C₁-C₁₀-alkylethers (b), di-C₁-C₄-alkyl-C₂-C₆-alkylene ethers and polyethers (c) and cyclic and acyclic acetals and ketals (e), even more preferred electrolyte composition (A) contains at least one aprotic organic solvent (i) selected from cyclic and acyclic organic carbonates (a) and most preferred electrolyte composition (A) contains at least two aprotic organic solvents (i) selected from cyclic and acyclic organic carbonates (a), in particular preferred electrolyte composition (A) contains at least one aprotic solvent (i) selected from cyclic organic carbonates and at least one aprotic organic solvent (i) selected from acyclic organic carbonates.

The aprotic organic solvents (a) to (j) may be partly halogenated, e.g. they may be partly fluorinated, partly chlorinated or partly brominated, preferably they may be partly fluorinated. “Partly halogenated” means, that one or more H of the respective molecule is substituted by a halogen atom, e.g. by F, Cl or Br. Preference is given to the substitution by F. The at least one solvent (i) may be selected from partly halogenated and non-halogenated aprotic organic solvents (a) to (j), i.e. the electrolyte composition may contain a mixture of partly halogenated and non-halogenated aprotic organic solvents.

Examples of suitable organic carbonates (a) are cyclic organic carbonates according to the general formula (a1), (a2) or (a3)

wherein

-   R^(a), R^(b) and R^(c) being different or equal and being     independently from each other selected from hydrogen; C₁-C₄-alkyl,     preferably methyl; F; and C₁-C₄-alkyl substituted by one or more F,     e.g. CF₃.

“C₁-C₄-alkyl” is intended to include methyl, ethyl, n-propyl, iso-propyl, n-butyl, iso-butyl, sec.-butyl and tert.-butyl.

Preferred cyclic organic carbonates (a) are of general formula (a1), (a2) or (a3) wherein R^(a), R^(b) and R^(c) are H. Examples are ethylene carbonate, vinylene carbonate, and propylene carbonate. A preferred cyclic organic carbonate (a) is ethylene carbonate. Further preferred cyclic organic carbonates (a) are difluoroethylene carbonate (a4) and monofluoroethylene carbonate (a5)

Examples of suitable acyclic organic carbonates (a) are dimethyl carbonate, diethyl carbonate, methylethyl carbonate and mixtures thereof.

In one embodiment of the invention the electrolyte composition (A) contains mixtures of acyclic organic carbonates (a) and cyclic organic carbonates (a) at a ratio by weight of from 1:10 to 10:1, preferred of from 3:1 to 1:1.

Examples of suitable acyclic di-C₁-C₁₀-alkylethers (b) are dimethylether, ethylmethylether, diethylether, diisopropylether, and di-n-butylether.

Examples of di-C₁-C₄-alkyl-C₂-C₆-alkylene ethers (c) are 1,2-dimethoxyethane, 1,2-diethoxyethane, diglyme (diethylene glycol dimethyl ether), triglyme (triethylene glycol dimethyl ether), tetraglyme (tetraethylene glycol dimethyl ether), and diethylene glycol diethyl ether.

Examples of suitable polyethers (c) are polyalkylene glycols, preferably poly-C₁-C₄-alkylene glycols and especially polyethylene glycols. Polyethylene glycols may comprise up to 20 mol % of one or more C₁-C₄-alkylene glycols in copolymerized form. Polyalkylene glycols are preferably dimethyl- or diethyl-end-capped polyalkylene glycols. The molecular weight M_(w) of suitable polyalkylene glycols and especially of suitable polyethylene glycols may be at least 400 g/mol. The molecular weight M_(w) of suitable polyalkylene glycols and especially of suitable polyethylene glycols may be up to 5 000 000 g/mol, preferably up to 2 000 000 g/mol.

Examples of suitable cyclic ethers (d) are tetrahydrofurane and 1,4-dioxane.

Examples of suitable acyclic acetals (e) are 1,1-dimethoxymethane and 1,1-diethoxymethane.

Examples for suitable cyclic acetals (e) are 1,3-dioxane and 1,3-dioxolane.

Examples of suitable ortho esters (f) are tri-C₁-C₄ alkoxy methane, in particular trimethoxymethane and triethoxymethane. Examples of suitable cyclic ortho esters (f) are 1,4-dimethyl-3,5,8-trioxabicyclo[2.2.2]octane and 4-ethyl-1-methyl-3,5,8-trioxabicyclo[2.2.2]octane.

Examples of suitable acyclic esters of carboxylic acids (g) are ethyl acetate, methyl butanoate, and esters of dicarboxylic acids like 1,3-dimethyl propanedioate. An example of a suitable cyclic ester of carboxylic acids (lactones) is γ-butyrolactone.

Examples of suitable cyclic and acyclic sulfones (h) are ethyl methyl sulfones, dimethyl sulfone and tetrahydrothiophene-S,S-dioxide.

Examples of suitable cyclic and acyclic nitriles and dinitriles (i) are adipodinitrile, acetonitrile, propionitrile, butyronitrile.

The water content of the inventive electrolyte composition is preferably below 100 ppm, based on the weight of the electrolyte composition, more preferred below 50 ppm, most preferred below 30 ppm. The water content may be determined by titration according to Karl Fischer, e.g. described in detail in DIN 51777 or ISO760: 1978.

The content of HF of the inventive electrolyte composition is preferably below 60 ppm, based on the weight of the electrolyte composition, more preferred below 40 ppm, most preferred below 20 ppm. The HF content may be determined by titration according to potentiometric or potentiographic titration method.

The inventive electrolyte composition (A) furthermore contains at least one conducting salt (ii). Electrolyte composition (A) functions as a medium that transfers ions participating in the electrochemical reaction taking place in an electrochemical cell. The conducting salt(s) (ii) present in the electrolyte are usually solvated in the aprotic organic solvent(s) (i). Preferably the conducting salt (ii) is a lithium salt. The conducting salt is preferably selected from the group consisting of

-   -   Li[F_(6−x)P(C_(y)F_(2y+1))_(x)], wherein x is an integer in the         range from 0 to 6 and y is an integer in the range from 1 to 20;         -   Li[B(R^(I))₄], Li[B(R^(I))₂(OR^(II)O)] and Li[B(OR^(II)O)₂]             wherein each R^(I) is independently from each other selected             from F, Cl, Br, I, C₁-C₄ alkyl, C₂-C₄ alkenyl, and C₂-C₄             alkynyl, wherein alkyl, alkenyl, and alkynyl may be             substituted by one or more OR^(III), wherein R^(III) is             selected from C₁-C₆ alkyl, C₂-C₆ alkenyl, and C₂-C₆ alkynyl,             and         -   (OR^(II)O) is a bivalent group derived from a 1,2- or             1,3-diol, a 1,2- or 1,3-dicarboxylic acid or a 1,2- or             1,3-hydroxycarboxylic acid, wherein the bivalent group forms             a 5- or 6-membered cycle via the both oxygen atoms with the             central B-atom;     -   LiClO₄; LiAsF₆; LiCF₃SO₃; Li₂SiF₆; LiSbF₆; LiAlCl₄,         Li[N(SO₂F)₂], lithium tetrafluoro (oxalato) phosphate; lithium         oxalate; and     -   salts of the general formula Li[Z(CnF_(2n+1)SO₂)_(m)], where m         and n are defined as follows:         -   m=1 when Z is selected from oxygen and sulfur,         -   m=2 when Z is selected from nitrogen and phosphorus,         -   m=3 when Z is selected from carbon and silicon, and         -   n is an integer in the range from 1 to 20.

Suited 1,2- and 1,3-diols from which the bivalent group (OR^(II)O) is derived may be aliphatic or aromatic and may be selected, e.g., from 1,2-dihydroxybenzene, propane-1,2-diol, butane-1,2-diol, propane-1,3-diol, butan-1,3-diol, cyclohexyl-trans-1,2-diol and naphthalene-2,3-diol which are optionally are substituted by one or more F and/or by at least one straight or branched non fluorinated, partly fluorinated or fully fluorinated C₁-C₄ alkyl group. An example for such 1,2- or 1,3-diole is 1,1,2,2-tetra(trifluoromethyl)-1,2-ethane diol.

“Fully fluorinated C₁-C₄ alkyl group” means, that all H-atoms of the alkyl group are substituted by F.

Suited 1,2- or 1,3-dicarboxylic acids from which the bivalent group (OR^(II)O) is derived may be aliphatic or aromatic, for example oxalic acid, malonic acid (propane-1,3-dicarboxylic acid), phthalic acid or isophthalic acid, preferred is oxalic acid. The 1,2- or 1,3-dicarboxylic acid are optionally substituted by one or more F and/or by at least one straight or branched non fluorinated, partly fluorinated or fully fluorinated C₁-C₄ alkyl group.

Suited 1,2- or 1,3-hydroxycarboxylic acids from which the bivalent group (OR^(II)O) is derived may be aliphatic or aromatic, for example salicylic acid, tetrahydro salicylic acid, malic acid, and 2-hydroxy acetic acid, which are optionally substituted by one or more F and/or by at least one straight or branched non fluorinated, partly fluorinated or fully fluorinated C₁-C₄ alkyl group. An example for such 1,2- or 1,3-hydroxycarboxylic acids is 2,2-bis(trifluoromethyl)-2-hydroxy-acetic acid.

Examples of Li[B(R^(I))₄], Li[B(R^(I))₂(OR^(II)O)] and Li[B(OR^(II)O)₂] are LiBF₄, lithium difluoro oxalato borate and lithium dioxalate borate.

Preferably the at least one conducting salt (ii) is selected from LiPF₆, LiBF₄, and LiPF₃(CF₂CF₃)₃, more preferred the conducting salt (ii) is selected from LiPF₆ and LiBF₄, and the most preferred conducting salt (ii) is LiPF₆.

The at least one conducting salt (ii) is usually present at a minimum concentration of at least 0.01 wt.-%, preferably of at least 1 wt.-%, and more preferred of at least 5 wt.-%, based on the total weight of the electrolyte composition. Usually the upper concentration limit for the at least one conducting salt (ii) is 25 wt.-%, based on the total weight of the electrolyte composition.

The inventive electrolyte composition (A) contains as component (iii) at least one compound of formula (I)

wherein

-   X is either carbon, S(O) or P(OR⁸), preferably X is carbon or S(O); -   n can be 0 or 1, preferably n is 1; -   R¹ is selected from C₁-C₆ alkyl, C₃-C₆ (hetero)cycloalkyl, C₂-C₆     alkenyl, C₃-C₆ (hetero)cycloalkenyl, C₂-C₆ alkynyl, C₅-C₇     (hetero)aryl, and C₆-C₁₃ aralkyl, wherein alkyl, (hetero)cycloalkyl,     aralkyl, alkenyl, (hetero)cycloalkenyl, alkynyl, and (hetero)aryl     may be substituted by one or more substituents selected from F, CN,     C₁-C₆ alkyl, C₃-C₆ (hetero)cycloalkyl, C₂-C₆ alkenyl, C₅-C₇     (hetero)aryl, OR⁵, C(O)R⁵, C(O)OR⁵, OC(O)R⁵, OC(O)OR⁵, OC(O)C(O)OR⁵,     OS(O)₂R⁵, and S(O)₂OR⁵; -   R², R³ and R⁴ are independently from each other selected from H,     C₁-C₆ alkyl, C₃-C₆ (hetero)cycloalkyl, C₂-C₆ alkenyl, C₃-C₆     (hetero)cycloalkenyl, C₂-C₆ alkynyl, C₅-C₇ (hetero)aryl, and C₆-C₁₃     aralkyl, C(O)R⁶, C(O)OR⁶, and S(O)₂OR⁶, wherein alkyl,     (hetero)cycloalkyl, aralkyl, alkenyl, (hetero)cycloalkenyl, alkynyl,     and (hetero)aryl may be substituted by one or more substituents     selected from F, CN, C₁-C₆ alkyl, C₃-C₆ (hetero)cycloalkyl, C₂-C₆     alkenyl, C₅-C₇ (hetero)aryl, OR⁷, C(O)R⁷, C(O)OR⁷, OC(O)R⁷,     OC(O)OR⁷, OC(O)C(O)OR⁷, OS(O)₂R⁷, and S(O)₂OR⁷;     -   or R² and R³ are combined with each other and form together with         the C—C single bond of the oxirane cycle a 5 to 7 membered         hydrocarbon cycle which is substituted by at least one group         selected from (O) and oxiranyl, and/or two adjacent C-atoms of         the hydrocarbon carbon cycle form together with an O-atom an         additional oxirane cycle; -   R⁵, R⁶, R⁷ and R⁸ are independently from each other selected from H,     C₁-C₆ alkyl, C₃-C₆ (hetero)cycloalkyl, C₂-C₆ alkenyl, C₃-C₆     (hetero)cycloalkenyl, C₂-C₆ alkynyl, C₅-C₇ (hetero)aryl, C₆-C₁₃     aralkyl, wherein alkyl, (hetero)cycloalkyl, aralkyl, alkenyl,     (hetero)cycloalkenyl, alkynyl, and (hetero)aryl may be substituted     by one or more substituents selected from F, CN, and oxiranyl.

For the sake of clarity, in the case that R² and R³ are combined to form together with the C—C single bond of the oxirane cycle a 5 to 7 membered hydrocarbon cycle R⁴ is defined as above.

The term “C₁-C₆ alkyl” as used herein means a straight or branched saturated hydrocarbon group with 1 to 6 carbon atoms having one free valence and includes, e.g., methyl, ethyl, n-propyl, isopropyl, n-butyl, sec-butyl, isobutyl, tert-butyl, n-pentyl, iso-pentyl, 2,2-dimethylpropyl, n-hexyl, iso-hexyl, 2-ethyl hexyl, n-heptyl, iso-heptyl, n-octyl, iso-octyl, n-nonyl, n-decyl and the like. Preferred are C₁-C₄ alkyl groups and most preferred are 2-propyl, methyl and ethyl.

The term “C₃-C₆ (hetero)cycloalkyl” as used herein means a cyclic saturated hydrocarbon group with 3 to 6 carbon atoms having one free valence wherein one or more C-atoms may be replaced by N, O or S. Examples of C₃-C₆ cycloalkyl include cyclopropyl, cyclobutyl, cyclopentyl and cyclohexyl, preferred is cyclohexyl. Examples of C₃-C₆ hetero cycloalkyl are oxiranyl and tetrahydrofuryl, preferred is oxiranyl (1,2-epoxy-ethyl). The term “oxirane cycle” as used herein means a substituted or unsubstituted oxirane cycle and includes oxiranyl, 1,1-oxiranediyl, and 1,2-oxiranediyl.

The term “C₂-C₆ alkenyl” as used herein refers to an unsaturated straight or branched hydrocarbon group with 2 to 6 carbon atoms having one free valence. Unsaturated means that the alkenyl group contains at least one C═C double bond. C₂-C₆ alkenyl includes for example ethenyl, 1-propenyl, 2-propenyl, 1-n-butenyl, 2-n-butenyl, iso-butenyl, 1-pentenyl, 1-hexenyl and the like. Preferred are C₂-C₄ alkenyl groups and in particular ethenyl and propenyl (1-propen-3-yl, allyl).

The term “C₃-C₆ (hetero)cycloalkenyl” as used herein refers to a cyclic unsaturated hydrocarbon group with 3 to 6 carbon atoms having one free valence wherein one or more C-atoms may be replaced by N, O or S. Unsaturated means that the cycloalkenyl group contains at least one C—C double bond. Examples of C₃-C₆ (hetero)cycloalkenyl are cyclopropenyl, cyclobutenyl, cyclopentenyl, and cyclohexenyl.

The term “C₂-C₆ alkynyl” as used herein refers to an unsaturated straight or branched hydrocarbon group with 2 to 6 carbon atoms having one free valence, wherein the hydrocarbon group contains at least one C—C triple bond. C₂-C₁₀ alkynyl includes for example ethynyl, 1-propynyl, 2-propynyl, 1-n-butinyl, 2-n-butynyl, iso-butinyl, 1-pentynyl, 1-hexynyl and the like. Preferred is C₂-C₄ alkynyl, in particular propynyl (1-propyn-3-yl, propargyl).

The term “C₅-C₇ (hetero)aryl” as used herein denotes an aromatic 5- to 7-membered hydrocarbon cycle having one free valence, wherein one or more C-atom may be replaced by N, O or S. An example of C₅-C₇ aryl is phenyl, examples of C₅-C₇ heteroaryl are pyrrolyl, furanyl, thiophenyl, pyridinyl, pyranyl, and thiopyranyl.

The term “C₇-C₁₃ aralkyl” as used herein denotes an aromatic 5- to 7-membered hydrocarbon cycle substituted by one or more C₁-C₆ alkyl. The C₇-C₁₃ aralkyl group contains in total 7 to 13 C-atoms and has one free valence. The free valence may be located at the aromatic cycle or at a C₁-C₆ alkyl group, i.e. C₇-C₁₃ aralkyl group may be bound via the aromatic part or via the alkyl part of the aralkyl group. Examples of C₇-C₁₃ aralkyl are methylphenyl, 1,2-dimethylphenyl, 1,3-dimethylphenyl, 1,4-dimethylphenyl, ethylphenyl, 2-propylphenyl, and the like.

The term “R² and R³ are combined with each other and form together with the C—C single bond of the oxirane cycle a 5 to 7 membered hydrocarbon cycle” is intended to mean that the 5- to 7-membered hydrocarbon cycle is formed by R² and R³ together with the C—C single bond of the oxirane cycle of general formula (I), i.e. the two C-atoms of the oxirane cycle are 2 members of the 5- to 7-membered hydrocarbon cycle.

According to one embodiment of the present invention the at least one compound of formula (I) is selected from compounds of formula (I)

wherein

-   R¹ is selected from C₁-C₆ alkyl; -   R² and R³ are independently from each other selected from H and     C₁-C₆ alkyl; and -   R⁴ is selected from H, C₁-C₆ alkyl, and C(O)OR⁷, wherein R⁷ is     preferably C₁-C₆ alkyl.

This embodiment includes compounds of formula (I) wherein R¹ is selected from C₁-C₆ alkyl and R² and R³ are H and R⁴ is H or C(O)OR⁷, wherein R⁷ is preferably C₁-C₆ alkyl, and compounds of formula (I) wherein R¹ is selected from C₁-C₆ alkyl and R², R³ and R⁴ are hydrogen.

According to another embodiment of the present invention the at least one compound of formula (I) is selected from compounds of formula (I) wherein X is a carbon and n equals 1.

Within this embodiment it is preferred if R¹ is selected from C₁-C₆ alkyl and R² and R³ are selected from hydrogen and C₁-C₆ alkyl, and R⁴ is selected from H, C₁-C₆ alkyl, and C(O)OR⁷, wherein R⁷ is preferably C₁-C₆ alkyl. This embodiment includes compounds of formula (I) wherein R¹ is selected from C₁-C₆ alkyl, R² and R³ are hydrogen, and R⁴ is C(O)OR⁷, wherein R⁷ is selected from C₁-C₆ alkyl, and compounds of formula (I) wherein R¹ is selected from C₁-C₆ alkyl, and R², R³ and R⁴ are hydrogen.

According to another embodiment of the invention electrolyte composition (A) contains compounds of formula (I) wherein X is S(O) and n equals 1. Preferably R¹ is selected from C₁-C₆ alkyl, R² and R³ are hydrogen and R⁴ is H or C(O)OR⁷ wherein R⁷ is selected from C₁-C₆ alkyl.

The compounds of formula (I) described above may contain only one oxirane cycle or at least two oxirane cycles.

The concentration of the at least one compound of formula (I) in the inventive electrolyte composition (A) usually is 0.001 to 10 wt.-%, preferred 0.01 to 2.5 wt.-%, more preferred 0.01 to 2 wt.-%, and most preferred 0.01 to 1.5 wt.-%, based on the total weight of the electrolyte composition (A).

The electrolyte composition (A) may contain at least one further additive (iv) which is selected from the group consisting of vinylene carbonate and its derivatives, vinyl ethylene carbonate and its derivatives, methyl ethylene carbonate and its derivatives, lithium (bisoxalato) borate, lithium difluoro (oxalato) borate, lithium tetrafluoro (oxalato) phosphate, lithium oxalate, 2-vinyl pyridine, 4-vinyl pyridine, cyclic exo-methylene carbonates, sultones, cyclic and acyclic sulfonates, cyclic and acyclic sulfites, cyclic and acyclic sulfinates, organic esters of inorganic acids, acyclic and cyclic alkanes having a boiling point at 1 bar of at least 36° C., and aromatic compounds, optionally halogenated cyclic and acyclic sulfonylimides, optionally halogenated cyclic and acyclic phosphate esters, optionally halogenated cyclic and acyclic phosphines, optionally halogenated cyclic and acyclic phosphites including, optionally halogenated cyclic and acyclic phosphazenes, optionally halogenated cyclic and acyclic silylamines, optionally halogenated cyclic and acyclic halogenated esters, optionally halogenated cyclic and acyclic amides, optionally halogenated cyclic and acyclic anhydrides, ionic liquids, and optionally halogenated organic heterocycles. The additive (iv) is preferably selected to be different from the compound selected as conducting salt (ii) present in the respective electrolyte composition (A). Preferably additive (iv) is also different from the at least one organic aprotic solvent (i) present in the respective electrolyte composition (A).

Preferred ionic liquids according to the present invention are selected from ionic liquids of formula [K]⁺[L]⁻ in which:

[K]⁺ denotes a cation, preferably reduction-stable, selected from the cation groups of the general formulae (II) to (IX)

wherein

-   R denotes H, C₁- to C₆-alkyl, C₂- to C₆-alkenyl, and phenyl,     preferably methyl, ethyl, and propyl; -   R^(A) denotes —(CH₂)_(s)—O—C(O)—R, —(CH₂)_(s)—C(O)—OR,     —(CH₂)_(s)—S(O)₂—OR, —(CH₂)_(s)—O—S(O)₂—R, —(CH₂)_(s)—O—S(O)₂—OR,     —(CH₂)_(s)—O—C(O)—OR, —(CH₂)_(s)—HC═CH—R, —(CH₂)_(s)—CN,

-   -   wherein individual CH₂ groups may be replaced by O, S or NR and         s=1 to 8, preferably s=1 to 3;

-   X^(A) denotes CH₂, O, S or NR^(B);

-   R^(B) denotes H, C₁- to C₆-alkyl, C₂- to C₆-alkenyl, phenyl, and     —(CH₂)_(s)—CN with s=1 to 8, preferably s=1 to 3; preferably R^(B)     is methyl, ethyl, propyl or H;     and

-   [L]⁻ denotes an anion selected from the group BF₄ ⁻, PF₆ ⁻,     [B(C₂O₄)₂]⁻, [F₂B(C₂O₄)]⁻, [N(S(O)₂F)₂]⁻,     [F_(p)P(C_(q)F_(2q+1))_(6−p)]⁻, [N(S(O)₂C_(q)F_(2q+1))₂]⁻,     [(C_(q)F_(2q+1))₂P(O)O]⁻, [C_(q)F_(2q+1)P(O)O₂]²⁻,     [OC(O)C_(q)F_(2q+1)]⁻, [OS(O)₂C_(q)F_(2q+1)]⁻;     [N(C(O)C_(q)F_(2q+1))₂]⁻;     [N(C(O)C_(q)F_(2q+1))(S(O)₂C_(q)F_(2q+1))]⁻;     [N(C(O)C_(q)F_(2q+1))(C(O))F]⁻; [N(S(O)₂C_(q)F_(2q+1))(S(O)₂F)]⁻;     [C(C(O)C_(q)F_(2q+1))₃]⁻, and [C(S(O)₂C_(q)F_(2q+1))₃N(SO₂CF₃)₂]⁻,     wherein p is an integer in the range from 0 to 6 and q is an integer     in the range from 1 to 20, preferably q is an integer in the ranger     from 1 to 4.

Preferred ionic liquids for use as additive (iv) are ionic liquids of formula [K][L] in which [K] is selected from pyrrolidinium cations of formula (II) with X is CH₂ and s is an integer in the range of from 1 to 3 and [L] is selected from the group consisting of BF₄ ⁻, PF₆ ⁻, [B(C₂O₄)₂]⁻, [F₂B(C₂O₄)]⁻, [N(S(O)₂F)₂]⁻, [N(SO₂C₂F₅)₂ ²]⁻, [F₃P(C₂F₅)₃]⁻, and [F₃P(C₄F₉)₃]⁻.

If one or more further additives (iv) are present in the electrolyte composition (A), the total concentration of further additives (iv) is at least 0.001 wt.-%, preferred 0.005 to 5 wt.-% and most preferred 0.01 to 2 wt.-%, based on the total weight of the electrolyte composition (A).

A further object of the present invention is the use of at least one compound of formula (I) as defined above as additive for electrolytes in electrochemical cells, preferably in lithium ion secondary electrochemical cells.

The compounds of general formula (I) are well-suited as film forming additives in electrochemical cells. The film may be formed on the anode and/or on the cathode. Preferably the compounds of general formula (I) are used as film forming additives in lithium ion secondary electrochemical cells, in particular as additives forming a film on the anode of lithium ion secondary electrochemical cells.

The compounds of general formula (I) are usually added to the electrolyte composition to yield a concentration of from is 0.001 to 10 wt.-%, preferred 0.01 to 2 wt.-% and most preferred 0.01 to 1.5 wt.-%, based on the total weight of the electrolyte composition (A).

Another object of the present invention is an electrochemical cell comprising

-   (A) the electrolyte composition as described above, -   (B) at least one cathode comprising at least one cathode active     material, and -   (C) at least one anode comprising at least one anode active     material.

Preferably the electrochemical cell is a secondary lithium ion electrochemical cell, i.e. secondary lithium ion electrochemical cell comprising a cathode comprising a cathode active material that can reversibly occlude and release lithium ions and an anode comprising a anode active material that can reversibly occlude and release lithium ions. The terms “secondary lithium ion electrochemical cell” and “(secondary) lithium ion battery” are used interchangeably within the present invention.

The at least one cathode active material preferably comprises a material capable of occluding and releasing lithium ions selected from lithiated transition metal phosphates and lithium ion intercalating transition metal oxides.

Examples of lithiated transition metal phosphates are LiFePO₄ and LiCoPO₄, examples of lithium ion intercalating transition metal oxides are transition metal oxides with layer structure having the general formula (X) Li_((1+z))[Ni_(a)CO_(b)Mn_(c)]_((1−z))O_(2+e) wherein z is 0 to 0.3; a, b and c may be same or different and are independently 0 to 0.8 wherein a+b+c=1; and −0.1≦e≦0.1, and manganese-containing spinels of general formula (XI) Li_(1+t)M_(2−t)O_(4−d) wherein d is 0 to 0.4, t is 0 to 0.4 and M is Mn and at least one further metal selected from the group consisting of Co and Ni, and Li_((1+g))[Ni_(h)CO_(i)Al_(j)]_((1−g))O_(2+k). Typical values for g, h, l, j and k are: g=0, h=0.8 to 0.85, i=0.15 to 0.20, j=0.02 to 0.03 and k=0.

In one preferred embodiment the cathode active material is selected from LiCoPO₄. The cathode containing LiCoPO₄ as cathode active material may also be referred to as LiCoPO₄ cathode. The LiCoPO₄ may be doped with Fe, Mn, Ni, V, Mg, Al, Zr, Nb, Tl, Ti, K, Na, Ca, Si, Sn, Ge, Ga, B, As, Cr, Sr, or rare earth elements, i.e., a lanthanide, scandium and yttrium. LiCoPO₄ with olivine structure is particularly suited according the present invention due to its high operating voltage (red-ox potential of 4.8 V vs. Li/Li⁺), flat voltage profile and a high theoretical capacity of about 170 mAh/g. The cathode may comprise a LiCoPO₄/C composite material. The preparation of a suited cathode comprising a LiCoPO₄/C composite material is described in Markevich et al., Electrochem. Comm., 2012, 15, 22-25.

In another preferred embodiment of the present invention the cathode active material is selected from transition metal oxides with layer structure having the general formula (X) Li_((1+z))[Ni_(a)Co_(b)Mn_(c)]_((1−z))O_(2+e) wherein z is 0 to 0.3; a, b and c may be same or different and are independently 0 to 0.8 wherein a+b+c=1; and −0.1≦e≦0.1. Preferred are transition metal oxides with layer structure having the general formula (X) Li_((1+z))[Ni_(a)CO_(b)Mn_(c)]_((1−z))O_(2+e) wherein z is 0.05 to 0.3, a=0.2 to 0.5, b=0 to 0.3 and c=0.4 to 0.8 wherein a+b+c=1; and −0.1≦e≦0.1. In one embodiment of the present invention, the transition metal oxides with layer structure of general formula (X) are selected from those in which [Ni_(a)Co_(b)Mn_(c)] is selected from Ni_(0.33)Co₀Mn_(0.66), Ni_(0.25)Co₀Mn_(0.75), Ni_(0.35)Co_(0.15)Mn_(0.5), Ni_(0.21)Co_(0.08)Mn_(0.71) and Ni_(0.22)Co_(0.12)Mn_(0.66), in particular preferred are Ni_(0.21)Co_(0.08)Mn_(0.71) and Ni_(0.22)Co_(0.12)Mn_(0.66). The transition metal oxides of general formula (X) are also called High Energy NCM (HE-NCM) since they have higher energy densities than usual NCMs. Both HE-NCM and NCM have operating voltage of about 3.3 to 3.8 V against Li/Li⁺, but high cut off voltages (>4.6 V) have to be used for charging HE-NCMS to actually accomplish full charging and to benefit from their higher energy density.

According to a further preferred embodiment of the present invention the cathode active material is selected from manganese-containing spinels of general formula (XI) Li_(1+t)M_(2−t)O_(4−d) wherein d is 0 to 0.4, t is 0 to 0.4 and M is Mn and at least one further metal selected from the group consisting of Co and Ni. An example of a suited manganese-containing spinel of general formula (XI) is LiNi_(0.5)Mn_(1.5)O_(4−d). These spinels are also called HE (high energy)-spinels.

Many elements are ubiquitous. For example, sodium, potassium and chloride are detectable in certain very small proportions in virtually all inorganic materials. In the context of the present invention, proportions of less than 0.5% by weight of cations or anions are disregarded, i.e. amounts of cations or anions below 0.5% by weight are regarded as non-significant. Any lithium ion-containing transition metal oxide comprising less than 0.5% by weight of sodium is thus considered to be sodium-free in the context of the present invention. Correspondingly, any lithium ion-containing mixed transition metal oxide comprising less than 0.5% by weight of sulfate ions is considered to be sulfate-free in the context of the present invention.

The cathode may further comprise electrically conductive materials like electrically conductive carbon and usual components like binders. Compounds suited as electrically conductive materials and binders are known to the person skilled in the art. For example, the cathode may comprise carbon in a conductive polymorph, for example selected from graphite, carbon black, carbon nanotubes, graphene or mixtures of at least two of the aforementioned substances. In addition, the cathode may comprise one or more binders, for example one or more organic polymers like polyethylene, polyacrylonitrile, polybutadiene, polypropylene, polystyrene, polyacrylates, polyvinyl alcohol, polyisoprene and copolymers of at least two comonomers selected from ethylene, propylene, styrene, (meth)acrylonitrile and 1,3-butadiene, especially styrene-butadiene copolymers, and halogenated (co)polymers like polyvinylidene chloride, polyvinyl chloride, polyvinyl fluoride, polyvinylidene fluoride (PVdF), polytetrafluoroethylene, copolymers of tetrafluoroethylene and hexafluoropropylene, copolymers of tetrafluoroethylene and vinylidene fluoride and polyacrylnitrile.

Furthermore, the cathode may comprise a current collector which may be a metal wire, a metal grid, a metal web, a metal sheet, a metal foil or a metal plate. A suited metal foil is aluminum foil.

According to one embodiment of the present invention the cathode has a thickness of from 25 to 200 μm, preferably of from 30 to 100 μm, based on the whole thickness of the cathode without the thickness of the current collector.

The anode (C) comprised within the lithium ion secondary battery of the present invention comprises an anode active material that can reversibly occlude and release lithium ions. In particular carbonaceous material that can reversibly occlude and release lithium ions can be used as anode active material. Carbonaceous materials suited are crystalline carbon such as a graphite material, more particularly, natural graphite, graphitized cokes, graphitized MCMB, and graphitized MPCF; amorphous carbon such as coke, mesocarbon microbeads (MCMB) fired below 1500° C., and mesophase pitch-based carbon fiber (MPCF); hard carbon and carbonic anode active material (thermally decomposed carbon, coke, graphite) such as a carbon composite, combusted organic polymer, and carbon fiber.

Further anode active materials are lithium metal, or materials containing an element capable of forming an alloy with lithium. Non-limiting examples of materials containing an element capable of forming an alloy with lithium include a metal, a semimetal, or an alloy thereof. It should be understood that the term “alloy” as used herein refers to both alloys of two or more metals as well as alloys of one or more metals together with one or more semimetals. If an alloy has metallic properties as a whole, the alloy may contain a nonmetal element. In the texture of the alloy, a solid solution, a eutectic (eutectic mixture), an intermetallic compound or two or more thereof coexist. Examples of such metal or semimetal elements include, without being limited to, titanium (Ti), tin (Sn), lead (Pb), aluminum, indium (In), zinc (Zn), antimony (Sb), bismuth (Bi), gallium (Ga), germanium (Ge), arsenic (As), silver (Ag), hafnium (Hf), zirconium (Zr) yttrium (Y), and silicon (Si). Metal and semimetal elements of Group 4 or 14 in the long-form periodic table of the elements are preferable, and especially preferable are titanium, silicon and tin, in particular silicon. Examples of tin alloys include ones having, as a second constituent element other than tin, one or more elements selected from the group consisting of silicon, magnesium (Mg), nickel, copper, iron, cobalt, manganese, zinc, indium, silver, titanium (Ti), germanium, bismuth, antimony and chromium (Cr). Examples of silicon alloys include ones having, as a second constituent element other than silicon, one or more elements selected from the group consisting of tin, magnesium, nickel, copper, iron, cobalt, manganese, zinc, indium, silver, titanium, germanium, bismuth, antimony and chromium.

A further possible anode active material is silicon which is able to intercalate lithium ions. The silicon may be used in different forms, e.g. in the form of nanowires, nanotubes, nanoparticles, films, nanoporous silicon or silicon nanotubes. The silicon may be deposited on a current collector. The current collector may be a metal wire, a metal grid, a metal web, a metal sheet, a metal foil or a metal plate. Preferred the current collector is a metal foil, e.g. a copper foil. Thin films of silicon may be deposited on metal foils by any technique known to the person skilled in the art, e.g. by sputtering techniques. One possibility of preparing Si thin film electrodes are described in R. Elazari et al.; Electrochem. Comm. (2012), 14, 21-24. It is also possible to use a silicon/carbon composite as anode active material according to the present invention.

Other possible anode active materials are lithium ion intercalating oxides of Ti.

Preferably the anode active material present in the inventive lithium ion secondary battery is selected from carbonaceous material that can reversibly occlude and release lithium ions, particularly preferred the carbonaceous material that can reversibly occlude and release lithium ions is selected from crystalline carbon, hard carbon and amorphous carbon, in particular preferred is graphite. In another preferred embodiment the anode active material present in the inventive lithium ion secondary battery is selected from silicon that can reversibly occlude and release lithium ions, preferably the anode comprises a thin film of silicon or a silicon/carbon composite. In a further preferred embodiment the anode active material present in the inventive lithium ion secondary battery is selected from lithium ion intercalating oxides of Ti.

The anode and cathode may be made by preparing an electrode slurry composition by dispersing the electrode active material, a binder, optionally a conductive material and a thickener, if desired, in a solvent and coating the slurry composition onto a current collector. The current collector may be a metal wire, a metal grid, a metal web, a metal sheet, a metal foil or a metal plate. Preferred the current collector is a metal foil, e.g. a copper foil or aluminum foil.

The inventive lithium ion batteries may contain further constituents customary per se, for example separators, housings, cable connections etc. The housing may be of any shape, for example cuboidal or in the shape of a cylinder, the shape of a prism or the housing used is a metal-plastic composite film processed as a pouch. Suited separators are for example glass fiber separators and polymer-based separators like polyolefin separators.

Several inventive lithium ion batteries may be combined with one another, for example in series connection or in parallel connection. Series connection is preferred. The present invention further provides for the use of inventive lithium ion batteries as described above in devices, especially in mobile devices. Examples of mobile devices are vehicles, for example automobiles, bicycles, aircraft, or water vehicles such as boats or ships. Other examples of mobile devices are those which are portable, for example computers, especially laptops, telephones or electrical power tools, for example from the construction sector, especially drills, battery-driven screwdrivers or battery-driven staplers. But the inventive lithium ion batteries can also be used for stationary energy stores.

The invention is illustrated by the examples which follow, which do not, however, restrict the invention.

1. Preparation of Compounds

Compound 1.

Racemic methyl glycidate 1 was prepared according to the following literature procedure: Facial Selectivity and Stereospecificity in the (4+3) Cycloaddition of Epoxy Enol Silanes. Lo, Brian; Chiu, Pauline, Organic Letters 2011, 13(5), pages 864 to 867.

Compound 2.

Methyl 2-methyl glycidate was prepared according to the following literature procedure: Efficient Epoxidation of Electron-Deficient Olefins with a Cationic Manganese Complex. Murphy, Andrew; Dubois, Geraud; Stack, T. D. P., Journal of the American Chemical Society 2003, 125(18), pages 5250 to 5251.

Compound 3.

Dimethyl 2,3-oxiranedicarboxylate 3 was prepared according to the following literature procedure: Preparation of dicarboxylate analogues of cerulenin. Moseley, Jonathan D.; Staunton, James, Journal of Heterocyclic Chemistry 2005, 42(5), pages 819 to 830.

Compound 4.

4,8-Dioxatricyclo[5.1.0.03,5]octane-2,6-dione 4 was prepared according to the following literature procedure: Acetal and ketal deprotection using montmorillonite K10: the first synthesis of syn-4,8-dioxatricyclo[5.1.0.03,5]-2,6-octanedione. Gautier, Elisabeth C. L.; Graham, Andrew E.; McKillop, Alexander; Standen, Stephen P.; Taylor, Richard J. K., Tetrahedron Letters 1997, 38(11), pages 1881 to 1884.

Compound 5.

Dimethyl-α,β-epoxyethylphosphonate 5 was prepared according to the following literature procedure: Preparation and ring opening reactions of α,β- and α,γ-epoxyalkylphosphonates. The proton magnetic resonance spectra of vicinally substituted ethyl- and propyl-phosphonates. Griffin, Claibourne E.; Kundu, S. K., Journal of Organic Chemistry 1969, 34(6), pages 1532 to 1539.

Compounds 1 to 5 are summarized in Table 1.

TABLE 1 Formulae of some compounds according to general formula (I) Compound Name Structure 1 Methyl glycidate

2 Methyl 2-methyl glycidate

3 Dimethyl 2,3- oxiranedicarboxylate

4 4,8-Dioxatricyclo [5.1.0.03,5] octane-2,6-dione

5 Dimethyl--epoxyethyl- phosphonate

2. Electrochemical Cells 2.1 Half Cells

The additives were investigated towards film formation in coin cells (half cells). Graphite-coated tapes (2.2 mAh/g) and metal lithium were used as electrodes. A glass-fiber filter separator (Whatmann GF/D) was used as the separator, which was soaked with 120 μl electrolyte. Coin type half cells (type 2032—Hohsen) were prepared with LiPF₆, which was dissolved in a 3:7 (by weight) mixture of ethylene carbonate/ethyl-methyl carbonate yielding a 1 M LiPF₆ solution (base electrolyte composition). 2 wt.-% of additive was added to the electrolyte mixture. All cells were assembled in an argon-filled glove box (Unilab, MBraun) having oxygen and water levels below 10 ppm. Afterwards the test cells were transferred to a battery test station. Electrochemical cycling was done using a Maccor battery test system. The half cells were initially held at open circuit potential for 6 hours and the graphite anodes were subsequently lithiated until the cell voltage reached 0.025 V. Afterwards the graphite anodes were delithiated until the cell voltage had a value of 1.5 V. This test was performed at 0.01 C rate. Measurements were carried out at room temperature (25° C.).

The reduction potentials obtained from differential capacity vs. voltage plots of coin cells (Li/graphite) containing the base electrolyte composition with 2 wt.-% electrolyte additive are summarized in Table 2.

2.2 Full Cells

Corresponding electrolyte additives were further investigated in full cells at ambient temperature (25° C.). The coin type cell contains a stainless steel spacer to contact the graphite anode and a stainless steel casing bottom to contact the backside of the cathode. The cathode material NCM 111 (LiNi_(0.33)Co_(0.33)Mn_(0.33)O₂) was used to make cathode tapes with a capacity of 2 mAh/cm². Graphite-coated tapes (capacity about 2.2 mAh/cm²) were used as anodes. A glass-fiber filter separator (Whatmann GF/D) was used as the separator, which was soaked with 120 μl electrolyte. Coin type full cells were prepared with LiPF₆ (Kanto Denka Koyo Co. Ltd), which was dissolved in a 3:7 (by weight) mixture of ethylene carbonate/ethyl methylcarbonate yielding a 1 M LiPF₆ solution (comparative sample). 0.5 wt.-% of a compound of formula (I) were added to the electrolyte mixture. All cells were assembled in an argon-filled glove box (Unilab, MBraun) having oxygen and water levels below 10 ppm. Afterwards the test cells were transferred to a battery test station. Electrochemical cycling (charging/discharging) was done using a Maccor battery test system. The full cells were initially held at open circuit potential for 6 hours and subsequently charged to 4.2 V. Afterwards the cells were discharged to a low voltage cutoff of 3.0 V. The first two cycles were performed at 0.1 C rate followed by cycling at 0.5 C (10 cycles), rate test up to 10 C and continuous cycling at 1 C between 3.0 and 4.2 V. All measurements were carried out in climate chambers. The internal resistance was determined by DC (discharge) pulse method (10 sec at 0.1 C, 10 sec at 1 C). The cell capacities obtained after 100 cycles and internal resistance values after 100 cycles of the NCM 111/graphite full cells are summarized in Table 2. The initial capacity of the cells was about 3.3 mAh.

TABLE 2 Test results Internal resist- Capacity ance Reduction after after potential 100 100 (V vs Li/Li⁺) cycles cycles Compound of formula (I) [V] [mAh] [Ω cm²]

1.15 3.28 33.66

1.35 3.26 33.20 None (comparative) 0.78 3.26 35.8 

The inventive electrolyte compositions containing compounds of formula (I) show lower or only slightly increased internal resistances in comparison to a similar electrolyte composition containing no compound of formula (I) and have a clearly higher reduction potential indicating the film formation on the anode. The reduction peak observed for the base electrolyte composition containing no additive is significantly reduced for the inventive electrolyte compositions. 

The invention claimed is:
 1. An electrolyte composition (A), comprising (i) an aprotic organic solvent; (ii) a conducting lithium salt; and (iii) a compound of formula (I)

wherein X is either carbon, S(O) or P(OR⁸); n is 0 or 1; R¹ is selected from the group consisting of C₁-C₆ alkyl, C₃-C₆ (hetero)cycloalkyl, C₂-C₆ alkenyl, C₃-C₆ (hetero)cycloalkenyl, C₂-C₆ alkynyl, C₅-C₇ (hetero)aryl, and C₆-C₁₃ aralkyl, wherein alkyl, (hetero)cycloalkyl, aralkyl, alkenyl, (hetero)cycloalkenyl, alkynyl, and (hetero)aryl are each optionally substituted by one or more substituents selected from the group consisting of F, CN, C₁-C₆ alkyl, C₃-C₆ (hetero)cycloalkyl, C₂-C₆ alkenyl, C₅-C₇ (hetero)aryl, OR⁵, C(O)R⁵, C(O)OR⁵, OC(O)R⁵, OC(O)OR⁵, OC(O)C(O)OR⁵, OS(O)₂R⁵, and S(O)₂OR⁵; R² is selected from the group consisting of H, C₁-C₆ alkyl, C₃-C₆ (hetero)cycloalkyl, C₂-C₆ alkenyl, C₃-C₆ (hetero)cycloalkenyl, C₂-C₆ alkynyl, C₅-C₇ (hetero)aryl, C₆-C₁₃ aralkyl, C(O)R⁶, C(O)OR⁶, and S(O)₂OR⁶, wherein alkyl, (hetero)cycloalkyl, aralkyl, alkenyl, (hetero)cycloalkenyl, alkynyl, and (hetero)aryl are each optionally substituted by one or more substituents selected from the group consisting of F, CN, C₁-C₆ alkyl, C₃-C₆ (hetero)cycloalkyl, C₂-C₆ alkenyl, C₅-C₇ (hetero)aryl, OR⁷, C(O)R⁷, C(O)OR⁷, OC(O)R⁷, OC(O)OR⁷, OC(O)C(O)OR⁷, OS(O)₂R⁷, and S(O)₂OR⁷; R³ and R⁴ are independently selected from the group consisting of H, C₁-C₆ alkyl, C₃-C₆ (hetero)cycloalkyl, C₂-C₆ alkenyl, C₃-C₆ (hetero)cycloalkenyl, C₂-C₆ alkynyl, C₅ or C₇ (hetero)aryl, C₆-C₁₃ aralkyl, C(O)R⁶, C(O)OR⁶, and S(O)₂OR⁶, wherein alkyl, (hetero)cycloalkyl, aralkyl, alkenyl, (hetero)cycloalkenyl, alkynyl, and (hetero)aryl are each optionally substituted by one or more substituents selected from the group consisting of F, CN, C₁-C₆ alkyl, C₃-C₆ (hetero)cycloalkyl, C₂-C₆ alkenyl, C₅-C₇ (hetero)aryl, OR⁷, C(O)R⁷, C(O)OR⁷, OC(O)R⁷, OC(O)OR⁷, OC(O)C(O)OR⁷, OS(O)₂R⁷ and S(O)₂O R⁷; or R² and R³ are combined with each other and form together with a C—C single bond of the oxirane cycle a 5 to 7 membered hydrocarbon cycle which is substituted by at least one selected from the group consisting of O and oxiranyl, and/or two adjacent C-atoms of the hydrocarbon carbon cycle form together with an O-atom an additional oxirane cycle; R⁵, R⁶, R⁷ and R⁸ are independently selected from the group consisting of H, C₁-C₆ alkyl, C₃-C₆ (hetero)cycloalkyl, C₂-C₆ alkenyl, C₃-C₆ (hetero)cycloalkenyl, C₂-C₆ alkynyl, C₅-C₇ (hetero)aryl, and C₆-C₁₃ aralkyl, wherein alkyl, (hetero)cycloalkyl, aralkyl, alkenyl, (hetero)cycloalkenyl, alkynyl, and (hetero)aryl are each optionally substituted by one or more substituents selected from the group consisting of F, CN, and oxiranyl; and (iv) optionally an additive.
 2. The electrolyte composition (A) according to claim 1, wherein R¹ is C₁-C₆ alkyl; R² and R³ are independently H or C₁-C₆ alkyl; and R⁴ is H, C₁-C₆ alkyl, or C(O)OR⁷.
 3. The electrolyte composition (A) according to claim 1, wherein R¹ is C₁-C₆ alkyl; and R², R³ and R⁴ are hydrogen.
 4. The electrolyte composition (A) according to claim 1, wherein X is a carbon; and n is
 1. 5. The electrolyte composition (A) according to claim 1, wherein X is carbon; n is 1; R¹ is C₁-C₆ alkyl; and R², R³ and R⁴ are hydrogen.
 6. The electrolyte composition (A) according to claim 1, wherein X is carbon; n is 1; R¹ is C₁-C₆ alkyl; R² and R³ are hydrogen; R⁴ is C(O)OR⁷; and R⁷ is C₁-C₆ alkyl.
 7. The electrolyte composition (A) according to claim 1, wherein X is S(O); and n is
 1. 8. The electrolyte composition (A) according to claim 1, wherein the compound of formula (I) comprises at least two oxirane cycles.
 9. The electrolyte composition (A) according to claim 1, wherein the aprotic organic solvent (i) is selected from (a) a cyclic or noncyclic organic carbonate, which is optionally partly halogenated, (b) a di-C₁-C₁₀-alkylether, which is optionally partly halogenated, (c) a di-C₁-C₄-alkyl-C₂-C₆-alkylene ether or polyether, which is optionally partly halogenated, (d) a cyclic ether, which is optionally partly halogenated, (e) a cyclic or acyclic acetal or ketal, which is optionally partly halogenated, (f) an orthocarboxylic acid ester, which is optionally partly halogenated, (g) a cyclic or noncyclic ester of a carboxylic acid, which is optionally partly halogenated, (h) a cyclic or noncyclic sulfone, which is optionally partly halogenated, (i) a cyclic or noncyclic nitrile or dinitrile, which is optionally partly halogenated, and (j) an ionic liquid, which is optionally partly halogenated.
 10. The electrolyte composition (A) according to claim 1, wherein the conducting lithium salt (ii) is selected from the group consisting of Li[F_(6-x)P(C_(y)F_(2y+1))_(x)], wherein x is an integer of from 0 to 6 and y is an integer of from 1 to 20; Li[B(R⁹)₄], Li[B(R⁹)₂(OR¹⁰O)] and Li[B(OR¹⁰O)₂] wherein each R⁹ is independently selected from the group consisting of F, Cl, Br, I, C₁-C₄ alkyl, C₂-C₄ alkenyl, and C₂-C₄ alkynyl, wherein alkyl, alkenyl, and alkynyl are each optionally substituted by one or more OR¹¹, wherein R¹¹ is selected from the group consisting of C₁-C₆ alkyl, C₂-C₆ alkenyl, and C₂-C₆ alkynyl, and (OR¹⁰O) is a bivalent group derived from a 1,2- or 1,3-diol, a 1,2- or 1,3-dicarboxlic acid or a 1,2- or 1,3-hydroxycarboxylic acid, wherein the bivalent group forms a 5- or 6-membered cycle via both oxygen atoms with a central B-atom; LiClO₄; LiAsF₆; LiCF₃SO₃; Li₂SiF₆; LiSbF₆; LiAlCl₄, Li[N(SO₂F)₂], lithium tetrafluoro (oxalato) phosphate; lithium oxalate; and a salt of formula Li[Z(C_(n)F_(2n+1)SO₂)_(m)], where m and n are defined as follows: m=1 when Z is oxygen or sulfur, m=2 when Z is nitrogen or phosphorus, m=3 when Z is carbon or silicon, and n is an integer of from 1 to
 20. 11. The electrolyte composition (A) according to claim 1, comprising the additive (iv) which is selected from the group consisting of vinylene carbonate and a derivative thereof, vinyl ethylene carbonate and a derivative thereof, methyl ethylene carbonate and a derivative thereof, lithium (bisoxalato) borate, lithium difluoro (oxalato) borate, lithium tetrafluoro (oxalato) phosphate, lithium oxalate, 2-vinyl pyridine, 4-vinyl pyridine, a cyclic exo-methylene carbonate, a sultone, an organic ester of an inorganic acid, an acyclic or a cyclic alkane having a boiling point at 1 bar of at least 36° C., and an aromatic compound, an optionally halogenated cyclic or acyclic sulfonylimide, an optionally halogenated cyclic or acyclic phosphate ester, an optionally halogenated cyclic or acyclic phosphine, an optionally halogenated cyclic or acyclic phosphite, an optionally halogenated cyclic or acyclic phosphazene, an optionally halogenated cyclic or acyclic silylamine, an optionally halogenated cyclic or acyclic halogenated ester, an optionally halogenated cyclic or acyclic amide, an optionally halogenated cyclic or acyclic anhydride, an ionic liquid and an optionally halogenated organic heterocycle.
 12. The electrolyte composition (A) according to claim 1, wherein a concentration of the compound of formula (I) is 0.001 to 10 wt.-% based on a total weight of the electrolyte composition (A).
 13. An electrochemical cell, comprising (A) the electrolyte composition according to claim 1, (B) a cathode comprising a cathode active material, and (C) an anode comprising an anode active material.
 14. The electrochemical cell according to claim 13, which is a secondary lithium ion battery.
 15. The electrolyte composition (A) according to claim 1, wherein R² is selected from the group consisting of C₁-C₆ alkyl, C₃-C₆ (hetero)cycloalkyl, C₂-C₆ alkenyl, C₃-C₆ (hetero)cycloalkenyl, C₂-C₆ alkynyl, C₅-C₇ (hetero)aryl, C₆-C₁₃ aralkyl, C(O)R⁶, C(O)OR⁶, and S(O)₂OR⁶, wherein alkyl, (hetero)cycloalkyl, aralkyl, alkenyl, (hetero)cycloalkenyl, alkynyl, and (hetero)aryl are each optionally substituted by one or more substituents selected from the group consisting of F, CN, C₁-C₆ alkyl, C₃-C₆ (hetero)cycloalkyl, C₂-C₆ alkenyl, C₅-C₇ (hetero)aryl, OR⁷, C(O)R⁷, C(O)OR⁷, OC(O)R⁷, OC(O)OR⁷, OC(O)C(O)OR⁷, OS(O)₂R⁷, and S(O)₂O R⁷, or R² and R³ are combined with each other and form together with a C—C single bond of the oxirane cycle a 5 to 7 membered hydrocarbon cycle which is substituted by at least one selected from the group consisting of 0 and oxiranyl, and/or two adjacent C-atoms of the hydrocarbon carbon cycle form together with an O-atom an additional oxirane cycle.
 16. The electrolyte composition (A) according to claim 1, wherein R³ and R⁴ are independently selected from the group consisting of H, C₂-C₆ alkyl, C₃-C₆ (hetero)cycloalkyl, C₂-C₆ alkenyl, C₃-C₆ (hetero)cycloalkenyl, C₂-C₆ alkynyl, C₅ or C₇ (hetero)aryl, C₆-C₁₃ aralkyl, C(O)R⁶, C(O)OR⁶, and S(O)₂OR⁶, wherein alkyl, (hetero)cycloalkyl, aralkyl, alkenyl, (hetero)cycloalkenyl, alkynyl, and (hetero)aryl are each optionally substituted by one or more substituents selected from the group consisting of F, CN, C₁-C₆ alkyl, C₃-C₆ (hetero)cycloalkyl, C₂-C₆ alkenyl, C₅-C₇ (hetero)aryl, OR⁷, C(O)R⁷, C(O)OR⁷, OC(O)R⁷, OC(O)OR⁷, OC(O)C(O)OR⁷, OS(O)₂R⁷, and S(O)₂OR⁷; or R² and R³ are combined with each other and form together with a C—C single bond of the oxirane cycle a 5 to 7 membered hydrocarbon cycle which is substituted by at least one selected from the group consisting of O and oxiranyl, and/or two adjacent C-atoms of the hydrocarbon carbon cycle form together with an O-atom an additional oxirane cycle.
 17. The electrolyte composition (A) according to claim 1, wherein R¹ is selected from the group consisting of C₁ or C₃-C₆ alkyl, C₃-C₆ (hetero)cycloalkyl, C₂-C₆ alkenyl, C₃-C₆ (hetero)cycloalkenyl, C₂-C₆ alkynyl, C₅-C₇ (hetero)aryl, and C₆-C₁₃ aralkyl, wherein alkyl, (hetero)cycloalkyl, aralkyl, alkenyl, (hetero)cycloalkenyl, alkynyl, and (hetero)aryl are each optionally substituted by one or more substituents selected from the group consisting of F, CN, C₁-C₆ alkyl, C₃-C₆ (hetero)cycloalkyl, C₂-C₆ alkenyl, C₅-C₇ (hetero)aryl, OR⁵, C(O)R⁵, C(O)OR⁵, OC(O)R⁵, OC(O)OR⁵, OC(O)C(O)OR⁵, OS(O)₂R⁵, and S(O)₂O R⁵.
 18. A method for preparing an electrolyte for an electrochemical cell, the method comprising: introducing a conducting lithium salt and a compound of formula (I)

into the electrolyte as an additive, wherein X is carbon, S(O) or P(OR⁸); n is 0 or 1; R¹ is selected from the group consisting of C₁-C₆ alkyl, C₃-C₆ (hetero)cycloalkyl, C₂-C₆ alkenyl, C₃-C₆ (hetero)cycloalkenyl, C₂-C₆ alkynyl, C₅-C₇ (hetero)aryl, and C₆-C₁₃ aralkyl, wherein alkyl, (hetero)cycloalkyl, aralkyl, alkenyl, (hetero)cycloalkenyl, alkynyl, and (hetero)aryl are each optionally substituted by one or more substituents selected from the group consisting of F, CN, C₁-C₆ alkyl, C₃-C₆ (hetero)cycloalkyl, C₂-C₆ alkenyl, C₅-C₇ (hetero)aryl, OR^(S), C(O)R⁵, C(O)OR⁵, OC(O)R⁵, OC(O)OR⁵, OC(O)C(O)OR⁵, OS(O)₂R⁵, and S(O)₂OR⁵; R² is selected from the group consisting of H, C₁-C₆ alkyl, C₃-C₆ (hetero)cycloalkyl, C₂-C₆ alkenyl, C₃-C₆ (hetero)cycloalkenyl, C₂-C₆ alkynyl, C₅-C₇ (hetero)aryl, C₆-C₁₃ aralkyl, C(O)R⁶, C(O)OR⁶, and S(O)₂OR⁶, wherein alkyl, (hetero)cycloalkyl, aralkyl, alkenyl, (hetero)cycloalkenyl, alkynyl, and (hetero)aryl are each optionally substituted by one or more substituents selected from the group consisting of F, CN, C₁-C₆ alkyl, C₃-C₆ (hetero)cycloalkyl, C₂-C₆ alkenyl, C₅-C₇ (hetero)aryl, OR⁷, C(O)R⁷, C(O)OR⁷, OC(O)R⁷, OC(O)OR⁷, OC(O)C(O)OR⁷, OS(O)₂R⁷, and S(O)₂OR⁷; R³ and R⁴ are independently selected from the group consisting of H, C₁-C₆ alkyl, C₃-C₆ (hetero)cycloalkyl, C₂-C₆ alkenyl, C₃-C₆ (hetero)cycloalkenyl, C₂-C₆ alkynyl, C₅ or C₇ (hetero)aryl, C₆-C₁₃ aralkyl, C(O)R⁶, C(O)OR⁶, and S(O)₂OR⁶, wherein alkyl, (hetero)cycloalkyl, aralkyl, alkenyl, (hetero)cycloalkenyl, alkynyl, and (hetero)aryl are each optionally substituted by one or more substituents selected from the group consisting of F, CN, C₁-C₆ alkyl, C₃-C₆ (hetero)cycloalkyl, C₂-C₆ alkenyl, C₅-C₇ (hetero)aryl, OR⁷, C(O)R⁷, C(O)OR⁷, OC(O)R⁷, OC(O)OR⁷, OC(O)C(O)OR⁷, OS(O)₂R⁷ and S(O)₂O R⁷; or R² and R³ are combined with each other and form together with a C—C single bond of an oxirane cycle a 5 to 7 membered hydrocarbon cycle which is substituted by at least one group selected from the group consisting of O and oxiranyl, and/or two adjacent C-atoms of the hydrocarbon carbon cycle form together with an O-atom an additional oxirane cycle; R⁵, R⁶, R⁷ and R⁸ are independently selected from the group consisting of H, C₁-C₆ alkyl, C₃-C₆ (hetero)cycloalkyl, C₂-C₆ alkenyl, C₃-C₆ (hetero)cycloalkenyl, C₂-C₆ alkynyl, C₅-C₇ (hetero)aryl, and C₆-C₁₃ aralkyl, wherein alkyl, (hetero)cycloalkyl, aralkyl, alkenyl, (hetero)cycloalkenyl, alkynyl, and (hetero)aryl are each optionally substituted by one or more substituents selected from the group consisting of F, CN, and oxiranyl. 